Despite the World Health Organization's labeling of tuberculosis (TB) as a global health emergency ten years ago, the problem of TB has worsened. This worsening is primarily a result of the growing HIV epidemic and the emergence of drug-resistant mutants of Mycobacterium tuberculosis. The emergence of M. tuberculosis strains resistant to two or more drugs causing multi-drug-resistant TB (MDR-TB) is threatening to render existing control strategies obselete. MDR-TB is at least 10 times more costly to treat than drug-sensitive TB, requiring more drugs and longer treatment times. In addition to being one of the most serious emerging infections in the world, MDR-TB is also recognized to be a potential threat as an agent of biological warfare and bioterrorism. An effective and improved vaccine to treat both TB and MDR-TB in normal and HIV-infected individuals is urgently needed. This program project application brings together investigators whose laboratories each have a long-standing interest in the pathogenesis and prevention of tuberculosis. The theme of the program is the molecular analysis of immunity to Mycobacterium tuberculosis. The program incorporates expertise both in the molecular microbiology of M. tuberculosis, and in the analysis of the host response to this organism. The overall long-term goal is to develop a more effective vaccine for the prevention of tuberculosis that will help to solve the current global health crisis due to this infection, and limit the emergence of new MDR-TB strains. The approach will investigate a new series of live attenuated M. tuberculosis strains that have been rendered avirulent and more immunogenic through a series of targeted mutations. Strategies are also proposed that will allow the rapid identification of genes in M. tuberculosis that can be targeted for deletion in order to further enhance the efficacy of live attenuated TB vaccines. Preclinical studies using the mouse model of tuberculosis will be carried out to assess safety and immunogenicity, and to determine the immunologic correlates of protection against infection and disease.